Onan MDJE 7.5 issues

Hi All I have a 1977 genny that I haven't used in a while so I decided to change all filters, oil and replaced the battery and the manual fuel pump with an electric unit. After bleeding the system I press the start button and it starts right up but dies as soon as I release the start button. Where should I start to diagnose the problem? Stop solenoid? Control board? What procedure should I use? Thanks

Check to make sure it's got oil pressure. If it does, you'll want to check the shutdown safety circuit (oil pressure and high temp switches). The safetys are bypassed during cranking, so if it's got no oil pressure or an issue in that circuit it'll start, but die when you let off the start button.

Is your main switch changed from shore power to generator?

pete

Yes I have oil pressure, Yes its on shore power.

If I'm remembering correctly, the safety switches on a the diesel J series Onans are normally open when running and close when there's an issue. Oil pressure is normally closed, opens once you have pressure. Temp switches are open and close on overheat. So you should be able to just disconnect the safeties to test and see if it stays running.

try switching main switch to generator

As soon as I start cranking the engine the oil pressure gauge starts to climb. Where are these safeties?

There may not be any power to your new fuel pump once you release the starter. Try and find a circuit that stays live whenever the power switch is on.

Xsbank I thought that too and even hot wired the fuel pump directly to the battery, same result.

Cliff Meima said:

try switching main switch to generator

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That's unlikely to make it stay running. And it's also a bad idea, as starting or stopping the generator with a load on it is not good for it.

Oh poop I thought that was easiest. I’ll bet its just a bleeding thing. Might have to undo an injector fitting to get the air out. Does the fuel pump actually rattle? Did you wire it backwards perhaps?

I cracked the bleedoff return line and let it pump so it's not air in the lines. I will try switching to the genny.

Okay I was at the boat today and tried the switch with no luck. I noticed that the solenoid doesn't activate during preheat but does during start.

When you push the start button and while it's down, you're bypassing the safeties. So if it dies when releasing the start button, it's gotta be in the safeties.

Great where are the safeties?

Should be at least 1 temperature switch on the cylinder heads, possibly an exhaust temperature switch (at or after the water injection point) too. And an oil pressure switch on the block.

To prove out there are no engine issues you can manually hold the run solenoid in the run position and start the unit. I have had a tie wrap on mine until I install my work around for the burned out emergency switch on my unit When it is time to stop the unit I simply push the control arm the opposite direction (it is spring loaded). Of course there are no safeties in circuit using this method.

There is a "speed switch" on the unit also that is known to be fussy but I am not sure of its function in the circuit. I would have to review that in the manual. It is in there for automatic operation I believe. Also the run solenoid has a pull coil and a hold coil. Not sure if the different coils are determined by the start switch or activated internally to the coil. Maybe others can jump in on those two items.